Soft silicone hydrogel contact lenses are increasingly becoming popular because of their high oxygen permeability and comfort. But, a silicone hydrogel material typically has a surface, or at least some areas of its surface, which is hydrophobic (non-wettable) and susceptible to adsorbing lipids or proteins from the ocular environment and may adhere to the eye. Thus, a silicone hydrogel contact lens will generally require a surface modification.
One of approaches for modifying the hydrophilicity of a relatively hydrophobic contact lens material is to apply at least one layer of a hydrophilic polymer comprising carboxyl groups to form layer-by-layer (LbL) coating onto silicone hydrogel contact lenses (see for example, U.S. Pat. No. 6,451,871, U.S. Pat. No. 6,719,929, U.S. Pat. No. 6,793,973, U.S. Pat. No. 6,884,457, U.S. Pat. No. 6,896,926, U.S. Pat. No. 6,926,965, U.S. Pat. No. 6,940,580, and U.S. Pat. No. 7,297,725, and U.S. Patent Application Publication Nos. US 2007/0229758A1, US 2008/0174035A1, and US 2008/0152800A1). Such an approach can be easily carried by a simple deposition technical, e.g., by simply contacting silicone hydrogel contact lenses with a solution of a hydrophilic polymer comprising carboxyl groups. Although such deposition technique can provide a cost effective process for rendering a silicone hydrogel material wettable, resultant LbL coatings may not be as durable and may have relatively high densities of negatively surface charges (e.g., carboxyl groups). To improve the durability, crosslinking of LbL coatings on contact lenses has been proposed in commonly-owned copending US patent application publication Nos. 2008/0226922 A1 and 2009/0186229 A1 (incorporated by reference in their entireties). However, crosslinked LbL coatings may still have relative high densities of surface charges (e.g., carboxyl groups). As discussed in US20120026458A1, contact lenses with a high surface concentration of negatively charged groups (e.g., carboxyl groups) are susceptible to high debris adhesion during patient handling, high protein adhesion during wearing (a majority of proteins in tears is believed to be positively charged), high deposition and accumulation of antimicrobials such polyhexamethylene biguanide (PHMB) present in contact lens care solutions. It would be desirable to have a low surface concentration of negatively charged groups (e.g., carboxyl groups) for silicone hydrogel contact lenses.
Therefore, there is still a need for a method for qualifying and quantifying carboxyl groups on lens surface and thereby for selecting a candidate polymer comprising carboxyl groups as a coating material for silicone hydrogel contact lenses as well as for optimizing a coating process for producing silicone hydrogel contact lenses with a coating thereon.